Color vision relies on the comparison in the brain of inputs from photoreceptors sensitive to different wavelengths of light. In the Drosophila retina, color detection is mediated within each ommatidium by different sets of photoreceptors that have different specializations and are distributed stochastically in the retina;p ommatidia appear to discriminate among short wavelengths while y ommatidia discriminate among longer wavelengths. The inner PR's project to the medulla part of the optic lobe where comparison between R7 and R8 (and perhaps between p and y PRs) occurs. We offer to investigate the color vision circuitry in the medulla: What types of neurons are contacted by R7 and R8;is there a difference between p and y target neurons? How do interneurons interconnect PRs and target neurons? The three specific aims will help us understand color vision processing in the Drosophila visual system. 1. Map medulla neurons. We have characterized the morphology and projection patterns of 15 types of medulla neurons involved in processing color vision. We will identify molecular markers for these neurons, establish their connectivity and the direction of information flow between them. We will investigate the type of neurotransmitters used by the PR target and by modulatory neurons. We will also attempt to uncover whether distinct maps independently process information from p and from y ommatidia, and whether a special map deals with comparison between p and y photoreceptors. 2. We will reconstruct the lineage of medulla cells and establish the logic of medulla development: Is it based on a columnar organization whereby one neuroblast gives rise to distinct neurons near each PR termination? Is there extensive migration of modulatory, non-columnar neurons? How many neuroblasts are required for the development of the medulla? 3. How does photoreceptor innervation or activity affect the establishment of the color vision network? We will genetically ablate or electrically silence each of the specific PR subtypes and study how medulla neurons are affected in their development and projection patterns, thus revealing possible inductive events from the retina to the optic lobes. To address the function of the genes that are specifically expressed in the various classes of medulla neurons, we will use loss- and gain of function experiments to define the role they play in cell fate determination. We hope to be able to build a precise anatomical description of the medulla that will complement classical work performed by Cajal and by Fischbach. In the future, this work will be followed by a functional analysis using behavior tests that we have developed, and through activity imaging and electrophysiology. This will provide precise information on the mechanisms by which Drosophila processes color information.